In the transmission of color television images, the color information of a color television image is generally transmitted separately from the luminance information, especially in order to preserve the black-and-white transmission compatibility of such a television image. The frequency bandwidth available for transmission of the color signals for such color information is considerably less than for transmission of the associated luminance signals. Moreover, during the transmission of color television images the color signal changes its transmission mode more often than the associated luminance signal. As a result, within a transmission of a color television image, the color signal is more often exposed to interference loci within the transmission path than is the luminance signal. In addition, these interference loci flatten out discontinuous changes, in particular in the transmitted color information in the transmission path, because of the much smaller transmission bandwidth, considerably more than the corresponding luminance discontinuities are flattened in the transmission path of the luminance signal. The flattening of originally discontinuous color transitions with a large difference in chroma causes, especially at a vertical color boundary of the original image, a disturbed color transition, which occurs in the form of a relatively broad, disturbing colored fringe, the color of which lies between the chroma of the two color areas contiguous to one another at the discontinuous boundary. Moreover, this disturbing color fringe is usually located to one side of the original discontinuous color boundary, so that a distorted color image, often located clearly to one side of the luminance contours, is reproduced.
Technical Bulletin TI 891023 of the Philips company, entitled "The TDA 4670 circuit for image signal reprocessing in television receivers," discloses a circuit for eliminating the aforesaid spurious colors in the color fringe of a vertical color boundary; this circuit contains, in the color difference transmission channels of a color television, differentiating edge detectors of an edge detector arrangement. Each edge detector generates at its output a detector signal, which depends on the type of signal edge detected, that is differentiated again in a downstream pulse shaper of the arrangement, and then conveyed to a switching circuit with a set switching threshold. The switching signals of the edge detector arrangement generated in this manner control an analog switch in the transmission line, at the output side of which is connected a storage capacitor. The analog switch is interrupted for the duration of the switching signal of the edge detector arrangement, so that while the analog switch is in the interrupted state, the storage capacitor determines the level of the output-side transmission signal, and keeps the transmission signal for that period at the level most recently applied before the analog switch opened. After the analog switch closes again, i.e. after the end of the signal edge, the signal on the output-side transmission line changes, with the relatively short charging time of the storage capacitor, to the level of the signal state after the signal edge, so that the slope of the corrected signal edge depends only on the short charging time of the storage capacitor. It has been found, however, that in many cases, especially when a video recorder is connected to the television, the disturbing spurious color fringes at vertical color boundaries are not eliminated, and furthermore a disturbed, weak color image is reproduced on the color television screen.
Unpublished German patent application P4132508.7 proposes a transmission arrangement for transmitting color television signals for reproducing a color television image on a screen, in which a delay line is arranged both in the transmission path of the luminance signal and in the transmission path of the associated color signal of the color television signals. The transmission path of the color signals also contains a signal switcher whose signal output is connected in one switch position of the signal switcher to the output of the delay line, and in the other switch position of the signal switcher to the input of the delay line. The signal switcher is controlled both by a color boundary detector detecting the color signal, and by a luminance discontinuity detector detecting the luminance signal. If no color discontinuity or luminance discontinuity is detected, the CSCC output signal is formed from the delayed color signal and the delayed luminance signal. During detection of a discontinuity in color or luminance, the signal output of the signal switcher is switched, during transmission of the delayed color discontinuity, to the input of the delay circuit in the transmission path of the color signal, at which the new color signal is already present behind the color discontinuity.
Although the spurious color band at a vertical color boundary is largely eliminated by the features just described, the color edge nevertheless still exhibits a disturbing jitter, which makes the color boundary unpleasantly unstable. This disturbance is especially pronounced with fairly noisy color television signals, or when the color television signal of one chroma changes, over a relatively flat signal edge, into another chroma, and the slope of the signal edge of the depicted color boundary fluctuates from line to line. The shape and amplitude of the detection signals that are generated in the processing circuits of the edge detectors depend on the nature of the detected signal edges, so that they result in very different detection times in terms of the beginning of the switching edge. This leads to a flickering PG,5 color boundary that is perceived as disturbing. This procedure is depicted schematically in diagrams (a) to (d) of FIG. 1. Depicted schematically in diagram (a) are the signal edges 1.1, 1.2, and 1.3 of three transmitted color signals 2.1, 2.2, and 2.3, each of which rises, within an edge duration Tf, from a level P1.1, P1.2, P1.3 representing a color F1, to a level P2.1, P2.2, P2.3 representing a color F2. Diagram (b) schematically depicts the three associated bell-shaped detection signals 3.1, 3.2 and 3.3 of an edge detector (not described further). Also drawn in diagram (b) is the reference level Ps at which the portion of a detector signal that exceeds this reference level generates, at the output of a threshold value switch of this detector arrangement, a switching signal depicted in diagram (c). In the exemplary embodiment depicted, the portion 4.3 of the detector signal 3.3 that exceeds the reference level Ps generates the switching signal 6.3. Since the three detection signals 3.1, 3.2, and 3.3 themselves have very different amplitudes and edge slopes, they pass through the reference level Ps of the detector arrangement at very different times t1, t2, or t3, although they begin at the same starting time t0. These aforesaid different times, however, define the switching edges 5.1, 5.2, and 5.3 for the detection time of the detected signal edges. Diagram (d) schematically depicts five video lines z1 to z5 of a video image, with a vertical color boundary K between the two areas with color F1, to the left of the color boundary, and F2, to the right of the color boundary K. The continually changing jitter resulting from the different switching edges 5.1, 5.2, and 5.3 is clearly evident in diagram (d).